In the middle of the Ocean II- cat collar

In the summer, my two little kitties started to show interest in birds and insects. One day, they came home with a bird. It was very hard to see a bird that couldn't fly anymore. Over a few weeks, my kitties came home with a few birds more. Some birds we could save, which was lucky. Some were not that lucky.
I decided to buy a cat collar for them to wear, when they go outside.
We didn't blame our our kitties, I know that it is their instinct. On the other hand, I feel sad for the birds.

I also sometimes take life's, sucking a spider into my vacuum cleaner, stepping on insects I never see and sometimes I eat both seafood and fish. So when I put a collar on my kitties, I think; what about myself?